1. Field of the Invention
The present invention relates to an optical sensor and an image forming apparatus.
2. Description of the Related Art
Conventionally, an image forming apparatus that performs image quality adjustment control such as process control based on specific conditions, e.g., immediately after the power is turned on or the accumulated number of printouts reaching a specific number, is known. In the image quality adjustment control, for example, a light-emitting element that is a light-emitting unit for an optical sensor emits light so that the emitted light is reflected on a bare surface portion (a portion where toner is not adhered) of an intermediate transfer belt as a detection target, and a light-receiving element that is a light-receiving unit for the optical sensor receives the light reflected and outputs an output signal (voltage) in response to the amount of light reflected. A reference toner image of a predetermined shape is then formed on a surface of a photosensitive element and is transferred onto the intermediate transfer belt. The light-emitting element then emits light so that the emitted light is reflected on the reference toner image as a detection target and the light-receiving element receives the reflected light and outputs the output signal in response to the light reflected. Thereafter, with the output signal of the bare surface of the intermediate transfer belt as a reference value, the output signal of the reference toner image is compared with the reference value to know the amount of toner adhered per unit area of the reference toner image. Based on the amount of toner adhered thus acquired, image forming conditions such as uniformly charged electrical potential of the photosensitive element, developing bias, writing light intensity for the photosensitive element, and a target control value of toner density of developer are adjusted so as to obtain a desired amount of toner adhered.
Such image quality adjustment control enables printouts in stable image density over an extended period of time.
The light-receiving element of the optical sensor may receive light other than the light reflected from a detection target such as the intermediate transfer belt or the reference toner image formed on the intermediate transfer belt. The output signal of the light-receiving element by the light other than the light reflected from the detection target is referred to as a crosstalk (or a crosstalk voltage, when the output signal is a voltage signal). Because the crosstalk deteriorates detecting accuracy of the detection target, it is desirable to keep the crosstalk as low level as possible.
The occurrence factors of the crosstalk include:
1. the light reflected from a case member covering a light-emitting element and a light-receiving element,
2. the light incident to the light-receiving element directly from the light-emitting element, and
3. the light reflected from a condenser lens or the like.
The first factor is suppressed, for example, by finishing the case member in non-glossy black, making it hard to reflect light.
The second factor is suppressed, as disclosed in Japanese Patent Application Laid-open No. 2005-24459, by providing the case member with a light blocking wall that blocks the light incident to the light-receiving element directly from the light-emitting element.
The third factor is suppressed by using a condenser lens of a higher transmittance.
However, even with those measures taken, it is not possible to eliminate the crosstalk completely, and thus the output signal of the detection target contains a noise signal (crosstalk) making it difficult to improve the detecting accuracy of the detection target.